DOI: 10.1148/radiol.2321030909
(Radiology 2004;232:181-186.)
© RSNA, 2004
Primary Malignant Melanoma of the Rectum: CT Findings in Eight Patients1
Kyoung Won Kim, MD,
Hyun Kwon Ha, MD,
Ah Young Kim, MD,
Tae Kyoung Kim, MD,
Jung-Sun Kim, MD,
Chang Sik Yu, MD,
Sung Won Park, MD,
Mi-Suk Park, MD,
Hye Jin Kim, MD,
Pyo-Nyun Kim, MD,
Jin Cheon Kim, MD and
Moon-Gyu Lee, MD
1 From the Departments of Radiology, Pathology, and Surgery, Asan Medical Center, University of Ulsan College of Medicine 388–1, Poongnap-dong, Songpa-ku, Seoul 138–736, Korea. Received June 9, 2003; revision requested August 21; revision received November 13; accepted December 9. Address correspondence to H.K.H. (e-mail: hkha@amc.seoul.kr).
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ABSTRACT
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PURPOSE: To retrospectively evaluate computed tomographic (CT) findings in patients with pathologically proved primary malignant melanoma of the rectum.
MATERIALS AND METHODS: CT scans of eight patients (three men and five women; age range, 38–74 years; mean age, 62 years) with histologically proved primary rectal malignant melanomas were retrospectively evaluated by two radiologists in consensus. Scans were evaluated for the involved site, approximate size and morphologic appearance of the primary mass, degree of perirectal infiltration, and presence or absence of lymphadenopathy, bowel obstruction, and distant metastases.
RESULTS: All of the tumors were located in the distal rectum just above the anal verge. The approximate average sizes of the tumors were as follows: length, 4.8 cm (range, 3.8–6.9 cm); width, 3.8 cm (range, 2.8–5.2 cm); and mean diameter, 4.3 cm (range, 3.3–5.8 cm). Tumors usually appeared as polypoid or fungating intraluminal masses (n = 7). Perirectal infiltration commonly extended to the pelvic side wall or the presacral space (n = 5). All eight patients had lymphadenopathy, frequently larger than 3 cm in diameter (n = 3), which most commonly involved the perirectal lymph node station (n = 7). There was no evidence of bowel obstruction in any of the patients. Distant metastasis involving the liver was noted in one patient.
CONCLUSION: On CT scans, primary rectal malignant melanomas appeared as bulky intraluminal fungating masses in the distal rectum, focally expanding and obscuring the lumen without causing obstruction, with perirectal infiltration and frequently enlarged lymph nodes.
© RSNA, 2004
Index terms: Colon neoplasms, 757.329 • Colon neoplasms, CT, 757.1211 • Melanoma, 757.329
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INTRODUCTION
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Malignant melanoma is known to be a frequent source of blood-borne metastases to the gastrointestinal tract (1–3), the majority of which involve the stomach and small bowel because of the rich blood supply; however, malignant melanoma is also known to occur primarily in the anal canal (4–8) and the esophagus (9–12). Primary malignant melanoma of the rectum is, however, an exceptionally rare condition, with documentation limited to case reports (13–16). The diagnosis of this condition has been the subject of considerable argument for many years, principally on the grounds that extension from a primary anal lesion is often impossible to exclude. While the term anorectal malignant melanoma was designed and commonly used to obviate the difficulty in defining the anorectal junction (4–8), there have been many cases reported in surgery, gastroenterology, and pathology literature that demonstrate that tumors arise in what is anatomically defined as the rectal mucosa, with no evidence of a synchronous lesion in the anal epithelium (13–16).
Whereas computed tomography (CT) is generally accepted as the principal imaging modality in the evaluation of abdominal and pelvic organs in patients with malignancies, to our knowledge there has been no report in the radiology literature regarding CT findings of primary malignant melanoma in the rectum or anorectum. During the past 7 years, we have evaluated eight patients with pathologically proved primary malignant melanoma of the rectum, which afforded us a unique opportunity to assess CT findings in patients with this rare tumor. Thus, the purpose of this study was to retrospectively evaluate CT findings in patients with pathologically proved primary malignant melanoma in the rectum.
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MATERIALS AND METHODS
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Patients
A computer search of the pathology, radiology, and medical records of our institution from January 1995 to December 2001 revealed 11 patients with pathologically proved primary malignant melanoma of the rectum. Our institution is one of the nationwide referral centers, although it is not specially designated for malignant melanoma. Primary rectal malignant melanoma was diagnosed when tumors were located in the rectal mucosa, with no evidence of a synchronous lesion in the anal epithelium, and when the patients had no history of melanoma and no evidence of melanoma involving the skin or eye at physical examination. All 11 patients underwent contrast material–enhanced CT. Among them, three patients were excluded because their CT scans were not available for review; thus, the remaining eight patients constituted our study population. The average patient age was 62 years (range, 38–74 years), and there were five women and three men. The study was approved by our institutional review board, and informed consent was not required.
CT Scanning and Image Evaluation
All CT scans were obtained with one of three commercially available helical CT scanners (Somatom Plus-S, Siemens, Erlangen, Germany [n = 2]; Somatom Plus 4, Siemens [n = 4]; Hispeed, GE Medical Systems, Milwaukee, Wis [n = 2]). Each patient received 100–120 mL of iopromide (Ultravist 300 or Ultravist 370; Schering, Berlin, Germany) through an 18- or 20-gauge angiographic catheter inserted in a forearm vein with a mechanical injector at a rate of 2.5–3.0 mL/sec. Examinations were performed 65–70 seconds after administration of contrast material with a collimation of 8–10 mm and a table pitch of 1:1.
CT scans were retrospectively reviewed by two experienced gastrointestinal radiologists (K.W.K., 7 years experience; H.K.H., 24 years experience) in consensus. The involved site of the rectum and the approximate sizes of the tumors, including mean diameters, were assessed. The morphologic appearance of the primary lesion was subjectively categorized as a polypoid or fungating intraluminal mass expanding the rectal lumen, an exophytic mass with or without compression of the parent rectal lumen, or a lesion without distinct mass but with rectal wall thickening. Rectal wall thickening was considered to exist when it was more than 1 cm thick. It was graded as concentric or eccentric. The contrast enhancement patterns of the lesions were grouped as either homogeneous or heterogeneous and were compared with the adjacent rectal wall. Perirectal infiltration was arbitrarily defined as being of grade 1, 2, or 3. Grade 1 perirectal infiltration was defined as no or minimal infiltration within the perirectal space, grade 2 as infiltration extending into the pararectal space but not into the pelvic side wall, and grade 3 as infiltration extending into the pelvic side wall or presacral space. Lymphadenopathy (ie, the location and size of enlarged nodes) was assessed with short-axis measurement. The presence or absence of colonic obstruction (ie, dilatation of the rectum and sigmoid colon above the tumor and fecal stasis) was assessed. The other abdominal organs, including the liver, were evaluated for presence or absence of distant metastases.
Record Review
The patients’ medical records were reviewed by one of the authors (H.J.K.) to determine each patient’s clinical presentation. All patients underwent sigmoidoscopy or colonoscopy with biopsy (Y.C.S., 10 years experience; K.J.C., 15 years experience), and six subsequently underwent abdominoperineal resection. The surgical, endoscopy, and pathology reports were reviewed to determine the findings.
Pathologic Review
In all patients, the specimens were also retrospectively reviewed by a gastrointestinal pathologist (J.S.K., 11 years experience) at our institution. The specimens were evaluated histopathologically for features of malignant melanoma and for presence or absence of melanin pigment in both the tumors and the epithelium. Immunohistochemistry was also performed with staining for S-100 and HMB-45, which are monoclonal antibodies that have a high sensitivity for reactive melanocytes and the majority of melanomas.
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RESULTS
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Clinical Findings
The principal clinical findings, together with the CT and histopathologic findings, are summarized in the Table.
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Symptoms and CT, Pathologic, and Histologic Findings in Eight Patients with Primary Malignant Melanoma of the Rectum
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Five of the eight patients presented with defecation difficulty (average duration, 6 months; range, 2–12 months). Patients also presented with rectal bleeding (n = 4), constipation (n = 3), decreased stool caliber (n = 2), weight loss (n = 2), and palpable inguinal mass (n = 1). Each patient had a palpable mass at digital rectal examination.
CT Findings
In all eight patients, the tumors were located in the distal rectum just above the level of the anal verge. The average sizes of the tumors were as follows: length, 4.8 cm (range, 3.8–6.9 cm); width, 3.8 cm (range, 2.8–5.2 cm); mean diameter, 4.3 cm (range, 3.3–5.8 cm). On CT scans, the primary lesions were usually seen as polypoid or fungating intraluminal masses (n = 7), which were often bulky in size and focally expanded the rectum without obstructing the lumen (n = 5) (Figs 1–3). The other tumor was seen as a lesion without a distinct mass but with uneven concentric rectal wall thickening (thickness, 1.4 cm) (Fig 4). The tumors showed either homogeneous contrast enhancement with hypoattenuation (n = 2) or heterogeneous contrast enhancement with intermediate to high attenuation (n = 6) when compared with the appearance of a normal rectal wall. Evidence of intratumoral hemorrhage or necrosis was not found in any of these patients. Perirectal infiltration was rated as grade 3 in five patients, grade 2 in one, and grade 1 in two.
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Figure 1a. Primary rectal malignant amelanotic melanoma in a 74-year-old woman. (a, b) Contrast material-enhanced transverse CT scans show a bulky polypoid intraluminal mass (arrows) that is focally expanding and obscuring the lumen in the distal rectum just above the anal verge. Perirectal infiltration (arrowheads) is mild and confined within the perirectal space. (c) Transverse CT scan obtained at the level of the upper rectum shows no evidence of colonic obstruction. The luminal caliber (arrowheads) of the upper rectum is normal. Also, note the left obturator lymphadenopathy (arrow).
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Figure 1b. Primary rectal malignant amelanotic melanoma in a 74-year-old woman. (a, b) Contrast material-enhanced transverse CT scans show a bulky polypoid intraluminal mass (arrows) that is focally expanding and obscuring the lumen in the distal rectum just above the anal verge. Perirectal infiltration (arrowheads) is mild and confined within the perirectal space. (c) Transverse CT scan obtained at the level of the upper rectum shows no evidence of colonic obstruction. The luminal caliber (arrowheads) of the upper rectum is normal. Also, note the left obturator lymphadenopathy (arrow).
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Figure 1c. Primary rectal malignant amelanotic melanoma in a 74-year-old woman. (a, b) Contrast material-enhanced transverse CT scans show a bulky polypoid intraluminal mass (arrows) that is focally expanding and obscuring the lumen in the distal rectum just above the anal verge. Perirectal infiltration (arrowheads) is mild and confined within the perirectal space. (c) Transverse CT scan obtained at the level of the upper rectum shows no evidence of colonic obstruction. The luminal caliber (arrowheads) of the upper rectum is normal. Also, note the left obturator lymphadenopathy (arrow).
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Figure 2a. Primary rectal malignant melanotic melanoma in a 57-year-old woman. (a, b) Contrast-enhanced transverse CT scans show a bulky intraluminal mass (arrows) that is focally expanding and obscuring the lumen in the distal rectum just above the anal verge. Perirectal infiltration (arrowheads) extends to the presacral space. (c) Transverse CT scan obtained at the level of the upper rectum shows no evidence of colonic obstruction (curved arrow) and also shows enlarged lymph nodes (arrowheads) in the perirectal and the right internal iliac lymph node stations. (d) Photograph of a gross specimen shows a large, lobulated intraluminal fungating mass in the distal rectum, abutting the anorectal junction (arrowheads). (e) Photomicrograph shows melanin pigmentation (arrowheads) scattered in the tumor, a finding that is typical of melanotic melanoma. (Hematoxylin-eosin stain; original magnification, x40.)
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Figure 2b. Primary rectal malignant melanotic melanoma in a 57-year-old woman. (a, b) Contrast-enhanced transverse CT scans show a bulky intraluminal mass (arrows) that is focally expanding and obscuring the lumen in the distal rectum just above the anal verge. Perirectal infiltration (arrowheads) extends to the presacral space. (c) Transverse CT scan obtained at the level of the upper rectum shows no evidence of colonic obstruction (curved arrow) and also shows enlarged lymph nodes (arrowheads) in the perirectal and the right internal iliac lymph node stations. (d) Photograph of a gross specimen shows a large, lobulated intraluminal fungating mass in the distal rectum, abutting the anorectal junction (arrowheads). (e) Photomicrograph shows melanin pigmentation (arrowheads) scattered in the tumor, a finding that is typical of melanotic melanoma. (Hematoxylin-eosin stain; original magnification, x40.)
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Figure 2c. Primary rectal malignant melanotic melanoma in a 57-year-old woman. (a, b) Contrast-enhanced transverse CT scans show a bulky intraluminal mass (arrows) that is focally expanding and obscuring the lumen in the distal rectum just above the anal verge. Perirectal infiltration (arrowheads) extends to the presacral space. (c) Transverse CT scan obtained at the level of the upper rectum shows no evidence of colonic obstruction (curved arrow) and also shows enlarged lymph nodes (arrowheads) in the perirectal and the right internal iliac lymph node stations. (d) Photograph of a gross specimen shows a large, lobulated intraluminal fungating mass in the distal rectum, abutting the anorectal junction (arrowheads). (e) Photomicrograph shows melanin pigmentation (arrowheads) scattered in the tumor, a finding that is typical of melanotic melanoma. (Hematoxylin-eosin stain; original magnification, x40.)
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Figure 2d. Primary rectal malignant melanotic melanoma in a 57-year-old woman. (a, b) Contrast-enhanced transverse CT scans show a bulky intraluminal mass (arrows) that is focally expanding and obscuring the lumen in the distal rectum just above the anal verge. Perirectal infiltration (arrowheads) extends to the presacral space. (c) Transverse CT scan obtained at the level of the upper rectum shows no evidence of colonic obstruction (curved arrow) and also shows enlarged lymph nodes (arrowheads) in the perirectal and the right internal iliac lymph node stations. (d) Photograph of a gross specimen shows a large, lobulated intraluminal fungating mass in the distal rectum, abutting the anorectal junction (arrowheads). (e) Photomicrograph shows melanin pigmentation (arrowheads) scattered in the tumor, a finding that is typical of melanotic melanoma. (Hematoxylin-eosin stain; original magnification, x40.)
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Figure 2e. Primary rectal malignant melanotic melanoma in a 57-year-old woman. (a, b) Contrast-enhanced transverse CT scans show a bulky intraluminal mass (arrows) that is focally expanding and obscuring the lumen in the distal rectum just above the anal verge. Perirectal infiltration (arrowheads) extends to the presacral space. (c) Transverse CT scan obtained at the level of the upper rectum shows no evidence of colonic obstruction (curved arrow) and also shows enlarged lymph nodes (arrowheads) in the perirectal and the right internal iliac lymph node stations. (d) Photograph of a gross specimen shows a large, lobulated intraluminal fungating mass in the distal rectum, abutting the anorectal junction (arrowheads). (e) Photomicrograph shows melanin pigmentation (arrowheads) scattered in the tumor, a finding that is typical of melanotic melanoma. (Hematoxylin-eosin stain; original magnification, x40.)
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Figure 3a. Primary rectal malignant melanotic melanoma in a 63-year-old man. (a) Contrast-enhanced transverse CT scan obtained at the level just above the anal verge shows a bulky intraluminal fungating mass (straight arrows) that is focally expanding and obscuring the distal rectal lumen, a perirectal lymphadenopathy (curved arrow) that is compressing the rectum, and a perirectal infiltration (arrowheads). (b) Transverse CT scan obtained at the level of the upper rectum shows no evidence of colonic obstruction (straight arrows) and also shows lymphadenopathy (curved arrow) in the right internal iliac lymph node station. Perirectal infiltration (arrowheads), extending to the presacral space, is also demonstrated.
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Figure 3b. Primary rectal malignant melanotic melanoma in a 63-year-old man. (a) Contrast-enhanced transverse CT scan obtained at the level just above the anal verge shows a bulky intraluminal fungating mass (straight arrows) that is focally expanding and obscuring the distal rectal lumen, a perirectal lymphadenopathy (curved arrow) that is compressing the rectum, and a perirectal infiltration (arrowheads). (b) Transverse CT scan obtained at the level of the upper rectum shows no evidence of colonic obstruction (straight arrows) and also shows lymphadenopathy (curved arrow) in the right internal iliac lymph node station. Perirectal infiltration (arrowheads), extending to the presacral space, is also demonstrated.
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Figure 4a. Primary rectal malignant melanoma in a 70-year-old woman. (a, b) Contrast-enhanced transverse CT scans show uneven concentric wall thickening (straight arrows) of the distal rectum just above the anal verge. Perirectal infiltration extends to the pararectal space (arrowheads). Lymphadenopathy (curved arrow) involving the perirectal lymph node station is also seen. (c) Transverse CT scan of the upper abdomen shows a large lobulated hepatic mass, which is suggestive of hepatic metastasis. The diagnosis of primary rectal melanoma and hepatic metastasis was assigned on the basis of histopathologic findings in biopsy specimens of the rectal and hepatic masses.
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Figure 4b. Primary rectal malignant melanoma in a 70-year-old woman. (a, b) Contrast-enhanced transverse CT scans show uneven concentric wall thickening (straight arrows) of the distal rectum just above the anal verge. Perirectal infiltration extends to the pararectal space (arrowheads). Lymphadenopathy (curved arrow) involving the perirectal lymph node station is also seen. (c) Transverse CT scan of the upper abdomen shows a large lobulated hepatic mass, which is suggestive of hepatic metastasis. The diagnosis of primary rectal melanoma and hepatic metastasis was assigned on the basis of histopathologic findings in biopsy specimens of the rectal and hepatic masses.
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Figure 4c. Primary rectal malignant melanoma in a 70-year-old woman. (a, b) Contrast-enhanced transverse CT scans show uneven concentric wall thickening (straight arrows) of the distal rectum just above the anal verge. Perirectal infiltration extends to the pararectal space (arrowheads). Lymphadenopathy (curved arrow) involving the perirectal lymph node station is also seen. (c) Transverse CT scan of the upper abdomen shows a large lobulated hepatic mass, which is suggestive of hepatic metastasis. The diagnosis of primary rectal melanoma and hepatic metastasis was assigned on the basis of histopathologic findings in biopsy specimens of the rectal and hepatic masses.
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Lymphadenopathy was noted in all eight patients and involved the perirectal (n = 7), internal iliac (n = 3), obturator (n = 2), and inguinal (n = 1) lymph node stations; the involved node was larger than 3 cm in diameter in three of the eight patients. There was a single enlarged lymph node in two patients, and there were multiple enlarged lymph nodes in six.
No patients had CT evidence of bowel obstruction. Distant metastasis was noted in one patient and involved the liver, which was seen as a large lobulated hepatic mass with heterogenous intratumoral enhancement (Fig 4).
Endoscopic Findings
In all eight patients, the tumors appeared as large polypoid or fungating masses involving the far distal rectum just above the dentate line. Surface ulceration was seen in three patients. One patient had multiple discrete satellite lesions adjacent to a large fungating mass in the distal rectum. Pigmentation of the tumor was only noted in two patients at endoscopy. One patient had patches of melanin pigmentation that were located a discrete distance from the primary tumor. Histologic examination of the endoscopic biopsy specimens revealed malignant melanoma in six patients; therefore, rectal malignant melanoma was diagnosed preoperatively on the basis of these findings. In the remaining two patients, the biopsy specimens revealed a poorly differentiated carcinoma that favored a diagnosis of adenocarcinoma (n = 1) and a poorly differentiated sarcoma that favored a diagnosis of stromal tumor (n = 1). Both patients were proved to have rectal malignant melanoma at histopathologic examination of the surgical specimens.
Histopathologic Findings
In the six patients in whom surgical specimens were available, each resected specimen showed a large fungating or ulcerofungating rectal mass. The average distance between the tumor and the dentate line was 1.3 cm (range, 0.2–3.8 cm). The mean maximal tumor diameter was 4.7 cm (range, 3.5–6.1 cm). Three tumors extended to the perirectal adipose tissue, one extended to the rectal muscle, and the other two were limited to the submucosa. At gross pathologic examination, four tumors were tanish brown in color, and the other two were grayish white. Two tumors had ulceration on their surfaces. In all six patients, surgery revealed lymphadenopathy involving the perirectal (n = 5), internal iliac (n = 2), and obturator (n = 1) nodes.
In all eight patients, histopathologic examination of the surgical or biopsy specimens revealed typical features of malignant melanoma, with pleomorphic lesions containing a high mitotic rate and prominent nucleoli (1). While six tumors showed melanin pigment at microscopic examination, however, the other two had no gross pathologic or microscopic evidence of pigmentation. All eight lesions were positive with immunohistolochemical HMB-45 staining, and seven were positive with S-100 staining, which supported the diagnosis of malignant melanoma (17).
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DISCUSSION
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While the anus itself is third behind the skin and the eyes as a source of primary malignant melanoma, this disease originates in the anorectal region, is relatively rare, and accounts for only 0.4%–1.6% of all melanomas and less than 1% of anal canal tumors (13). Primary malignant melanoma that arises from the true anatomic rectal mucosa is extremely rare (13–16). For this reason, when malignant melanoma is found in the rectum, it is important to rule out a metastatic implant or an extension from a primary anal lesion as the cause of the rectal lesion. In other words, primary rectal malignant melanoma can be diagnosed only in patients who have no history of melanoma and no evidence of melanoma involving the skin, eyes, or anus. The origin of melanoma within the rectal mucosa has been disputed, and two possibilities have been postulated for the cause, including some form of heterotopia via differentiation from a primitive stem cell, with migration from the anal mucosa into the rectum (14).
Clinicopathologically, primary rectal malignant melanomas are usually diagnosed in elderly patients (14). Reportedly, rectal bleeding and tenesmus are common clinical symptoms (15); however, patients may present with diverse symptoms, and the diagnosis is rarely indicated by clinical findings. In some instances, these lesions appear as darkly pigmented masses at endoscopy; thus, the examiner might suggest the correct diagnosis (1,14). More frequently, however, pigmentation is not apparent at visual inspection; sometimes it is completely absent in amelanotic melanomas, even when viewed with microscopy. In these cases, immnunohistochemical studies that do not depend on the presence of the melanin pigment are required to confirm the diagnosis. Although quite sensitive, antibodies to S-100 protein are not melanoma-specific and react with a diverse set of mesenchymal tumors, as well as a subset of carcinomas. On the other hand, HMB-45, which is another monoclonal antibody, is specific for melanocytic tumors. Although HMB-45 is not useful in distinguishing benign and malignant melanocytic proliferations, the absence of false-positive reactions with other malignant tumors allows one to identify with certainty any undifferentiated malignancies that react with this antibody as melanomas (17).
In our series, the most common symptom at presentation was defecation difficulty (63%), followed by rectal bleeding (50%), constipation (38%), decreased stool caliber (25%), weight loss (25%), and palpable inguinal mass (13%). In two patients, the diagnosis of malignant melanoma was suggested at endoscopy on the basis of the dark-brown pigmentation of the lesion; however, this diagnosis could be assigned only after histopathologic examination of the biopsy or surgical specimens of the other six patients. An incorrect diagnosis of poorly differentiated carcinoma or poorly differentiated sarcoma was assigned at histopathologic examination of the biopsy specimens of these two patients; however, the correct diagnosis was assigned postoperatively in both patients. On the basis of the findings of histopathologic examination, six tumors showed microscopic melanin pigmentation, while the other two were determined to be amelanotic melanomas. In these two tumors, the diagnosis of malignant melanoma was possible because the results of immunohistolochemical staining with HMB-45 and S-100 were positive.
On the other hand, when primary rectal malignant melanomas were retrospectively reviewed in our series, they had a strikingly similar morphologic appearance, even though we had a limited number of cases. On CT scans, these tumors usually appeared as polypoid or fungating intraluminal masses (seven of 8 patients, 88%); they appeared as bulky intraluminal fungating masses that focally expanded and obscured the rectal lumen without causing colonic obstruction in five (63%) patients. This finding was correlated with the findings of endoscopic or histopathologic examinations, which showed that the tumors were located in the distal rectum just above the dentate line, without evidence of a synchronous lesion in the anal epithelium. On CT scans, these tumors usually appeared as polypoid or fungating intraluminal masses (seven of eight patients, 88%); however, they also commonly appeared as bulky intraluminal fungating masses that focally expanded and obscured the rectal lumen without causing colonic obstruction (five of eight patients, 63%). Such an appearance is somewhat similar to that of primary malignant melanomas of the esophagus and is probably related to the growth pattern of these tumors. For example, it has been reported that the tumor tends to grow intraluminally along the longitudinal axis of the bowel in patients with primary esophageal malignant melanoma, which leads to the development of a polypoid mass that widens the lumen as it grows (9). One patient had a tumor that appeared to be a lesion with concentric rectal wall thickening but without a distinct mass on CT scans; however, it was unevenly thickened (up to 1.4 cm in thickness), and was diagnosed as a fungating intraluminal mass at endoscopy.
Perirectal infiltration commonly extended to the pelvic side wall or to the presacral space (grade 3, 63%). Among the five patients in which this occurred, three underwent abdominoperineal resection and were proved to have diseases with extension to perirectal adipose tissue; the other two patients did not undergo surgery. All eight patients had evidence of lymph node metastases on CT scans; this was also surgically and histopathologically proved in the six patients who underwent surgery. The metastatic lymphadenopathy most commonly involved the perirectal lymph nodes and was larger than 3 cm in diameter in three (38%) of our patients.
A diagnosis of primary rectal malignant melanoma depends primarily on the histopathologic and endoscopic findings. When radiologists encounter a large intraluminal rectal mass with prominent perirectal infiltration and perirectal and regional lymphadenopathy on CT scans, the major consideration in the differential diagnosis is adenocarcinoma, because it is the most common malignancy of the rectum. It is uncommon for adenocarcinoma of the rectum to appear as an expansile rectal mass without causing an obstruction, however, because this tumor tends to infiltrate, narrow the lumen rather than expand it, and cause obstruction.
There are some limitations to our study. The predominant limiting factor, small sample size, was due to the rarity of the tumor. Also, although strikingly similar CT patterns were noted, we found nothing specific about melanoma that would suggest the diagnosis at prospective interpretation of CT scans. Another limitation is the 8–10-mm section thickness; however, this may not be a major limitation since the lesions were bulky.
In conclusion, although rare, primary malignant melanomas do occur in the distal rectum and yield strikingly similar patterns on CT scans; these tumors appear as a bulky intraluminal fungating mass in the distal rectum, focally expanding and obscuring the lumen without causing obstruction, with prominent perirectal infiltration and lymphadenopathy that is frequently large in size.
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ACKNOWLEDGMENTS
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We thank Bonnie Hami, MA, for her editorial assistance.
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FOOTNOTES
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Author contributions: Guarantor of integrity of entire study, M.G.L.; study concepts, K.W.K., H.K.H.; study design, K.W.K., A.Y.K., T.K.K.; literature research, H.J.K.,S.W.P.; clinical studies, K.W.K., C.S.Y., J.S.K., J.C.K.; data acquisition, H.J.K., S.W.P.; data analysis/interpretation, K.W.K., H.K.H., J.S.K., C.S.Y.; manuscript preparation, K.W.K., M.S.P.; manuscript definition of intellectual content, A.Y.K., T.K.K.; manuscript editing, K.W.K.; manuscript revision/review, P.N.K., M.G.L.; manuscript final version approval, H.K.H.
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ABSTRACT
INTRODUCTION
